Investigations of mysterious geoglyphs reveal long history of human impacts on Amazon rainforest

The Amazonian rainforest was transformed more than two thousand years ago by ancient people who built hundreds of large, mysterious earthworks, according to findings by experts, who have provided new evidence for how indigenous people lived in the Amazon before European people arrived in the region.

Amazon earthworks courtesy of Diego GurgelThe international research team, from Brazil and the UK, which included Dr Neil Loader and Emeritus Professor Alayne Street- Perrott from Swansea University’s Department of Geography, College of Science, published its findings in the Proceedings of the National Academy of Sciences of the USA (PNAS).

Their research investigated the ditched enclosures, in Acre state in the western Brazilian Amazon, which were concealed for centuries by bamboo-dominated rainforest until modern deforestation allowed the discovery of more than 450 of these large geometrical geoglyphs.

Picture courtesy of Diego Gurgel.

The function of these mysterious sites is still little understood.  They are unlikely to be villages, since archaeologists recover very few artefacts during excavation and the layout doesn’t suggest that they were built for defensive reasons. Instead it is thought they were used only sporadically, perhaps as ritual gathering places.

Amazon earthworks courtesy of Diego Gurgel‌The structures are ditched enclosures scattered over roughly 13,000 km2. Theirdiscovery challenges assumptions that the rainforest ecosystem has been untouched by humans.

Dr Jenny Watling, of Exeter University who led the study said: “The fact that these sites lay hidden for centuries beneath mature rainforest really challenges the idea that Amazonian forests are ‘pristine ecosystems`.

Picture courtesy of Diego Gurgel

“We immediately wanted to know whether the region was already forested when the geoglyphs were built, and to what extent people impacted the landscape to build these earthworks.”

Using state-of-the-art methods, the team members were able to reconstruct 6000 years of vegetation and fire history around two geoglyph sites. They found that humans heavily altered bamboo forests for millennia and small, temporary clearings were made to build the geoglyphs.

‌Dr Loader carried out an analysis of carbon-isotopes at Swansea University which suggests that the geoglyphs were not constructed in open, grassy savanna as might have been expected. Dr Loader said: “The indications are that the geoglyphs were constructed amongst taller vegetation. So, unlike the towering Maya pyramids of Central America, they were likely not visible above the forest canopy, and this raises questions about their purpose.”

The research suggests that instead of burning large tracts of forest – either for geoglyph construction or agricultural practices – people transformed their environment by concentrating on economically valuable tree species such as palms, creating a kind of ‘prehistoric supermarket’ of useful forest products. The team found tantalizing evidence to suggest that the biodiversity of some of Acre’s remaining forests may have a strong legacy of these ancient ‘agroforestry’ practices.

Despite the huge number and density of geoglyph sites in the region, the researchers were certain that Acre’s forests were never cleared as extensively, or for as long, as they have been in recent years.

Amazon earthworks courtesy of Edison Caetano‌Dr Watling said: “Our evidence that Amazonian forests have been managed by indigenous peoples long before European Contact should not be cited as justification for the destructive, unsustainable, land-use practised today. It should instead serve to highlight the ingenuity of past subsistence regimes that did not lead to forest degradation, and the importance of indigenous knowledge for finding more sustainable land-use alternatives”.

Picture courtesy of Edison Caetano


The research published in the Proceedings of the National Academy of Sciences of the USA  involved researchers from the universities of Exeter, Swansea, and Reading (UK), São Paulo, Belém and Acre (Brazil). The research was funded by the Arts and Humanities Research Council, National Geographic, and the Natural Environment Research Council Radiocarbon Facility.

To conduct the study, the team extracted soil samples from a series of pits dug within and outside of the geoglyphs. From these soils, they analysed ‘phytoliths’, a type of microscopic plant fossil made of silica, to reconstruct ancient vegetation; charcoal quantities, to assess the amount of ancient forest burning; and carbon stable isotopes, to indicate the types of vegetation growing there in the past.

Read the research paper here